Local area network (LAN) systems are well known in the art. Cellular LAN systems, which use the technology of cellular radio systems to achieve a wireless LAN system, are also well known. Examples of such systems include the following:
Altair Plus II, using microwave technology, commercially available from Motorola; PA1 WaveLAN, using spread spectrum technology, commercially available from A T & T (NCR); PA1 Collaborative TX and Collaborative Access, using diffuse IR technology, commercially available from Photonics; PA1 RangeLAN2/ISA and RangeLAN2/Access Point, using spread spectrum technology, commercially available from Proxim; PA1 Freeport Hub and Freeport Tex., using spread spectrum technology, commercially available from Windata; and PA1 PCMCIA Netwave and Netwave Access Point, using spread spectrum technology, commercially available from Xircom.
Cellular LAN systems typically comprise a plurality of access points and a plurality of mobile stations, and may also comprise one or more non-mobile stations. The term "access point", as used throughout the present specification and claims, refers to a generally non-mobile station operatively connected to a conventional LAN, and also capable of being in radio contact with one or more mobile stations, as well as one or more non-mobile stations, at any given time. Each access point provides one or more mobile stations with access to a conventional LAN by receiving messages from the one or more mobile stations and forwarding the messages to the conventional LAN and, conversely, by receiving messages from the conventional LAN and forwarding the messages to a mobile station. Each access point typically also provides relay of messages between one mobile station and another mobile station, without necessarily using the conventional LAN.
As a mobile station moves, it may reach a limit, typically a distance limit from the antenna of the access point, for acceptable radio communication for a given access point. When such a limit is reached, it is desirable to hand off the mobile station from the previous access point to a new access point. It is thus necessary to determine when such a hand off should take place. In cellular LAN systems various techniques are used to determine when a mobile station should be handed off from one access point to another access point. For example, a mobile station may be handed off when a connection to an access point is being lost. Alternatively, periodic searching of a plurality of access points may be performed in order to find the best access point.
Technology relevant to cellular LAN systems is described in the following U.S. patents: U.S. Pat. Nos. 5,123,029; 5,324,925; 5,179,569; 5,307,372; 5,361,401; 5,311,504; 5,381,443; 5,361,401; 4,653,068; 4,606,040; 4,606,041; 4,630,282; 5,210,753; and 5,384,826.
U.S. Pat. No. 5,371,738 to Moelard et al describes a wireless local area network system in which communications are switched from a first base station to a second base station based in response to a decision at a mobile wireless station. Moelard does not describe a specific method by which the decision is made.
A draft protocol for wireless LAN communication is described in Draft Standard IEEE 802.11, Wireless LAN, Institute of Electrical and Electronics Engineers, Inc., 345 E. 47th Street, New York, N.Y. 10017, U.S.A., published Jan. 25, 1996.
The disclosures of the above publications and of the publications cited therein are hereby incorporated by reference. The disclosures of all publications mentioned in this specification and of the publications cited therein are hereby incorporated by reference.